Method of melting cast iron

ABSTRACT

AN IMPROVED METHOD OF MELTING CAST IRON AT HIGH TEMPERATURE TO ACCOMPLISH MANY OF THE ADVANTAGES OF HIGH TEMPERATURE MELTING INCLUDING INCREASED FLUDITY WHILE OVERCOMING THE DISADVANTAGES OF HIGH TEMPERATURE MELTING, FOR EXAMPLE, REDUCED UNDERCOOLING, BY THE EXPEDIENT OF ADDING A SMALL BUT EFFECTIVE AMOUNT OF A SILICON CARBIDE TO THE CAST IRON MELT.

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2600F 2700F 2aoor 29oor 3000"? INVENTOR 4-44. We ru ATTORNEYS United States Patent 3,764,298 METHOD OF MELTING CAST IRON William H. Moore, Purchase, N.Y., assignor to Meehanite Metal Corporation Filed Sept. 2,1969, Ser. No. 854,606 Int. Cl. C21b 15/02; C21c /52 U.S. C]. 7512 4 Claims ABSTRACT OF THE DISCLOSURE My invention relates to an improved method of melting cast iron in the electric furnace and, more particularly, to a method of melting at high temperatures.

In the melting of cast iron in the electric furnace, it is customary to exert close control on the temperature to which the bath is heated, because excessive superheat, above the solidification temperature of cast iron, will produce excessive chill or carbide hardness in the metal or, alternatively, will result in graphite of the undercooled and interdendritic variety. This limitation of melting temperature does not allow full utilization of the outstanding characteristic of the electric furnace, viz., the fact that high pouring temperatures and increased metal fluidity may be obtained quite readily.

My invention is based on the discovery that small portions of silicon carbide may be added to the bath and will completely prevent the deleterious effects of high superheating temperatures.

An object of this invention is to provide a means whereby cast iron may be heated to a high pouring temperature, without ill effect.

A further object is to allow the production of cast iron of normal graphite structure at high superheat temperatures.

A further object is to improve the fluidity of a cast iron melt.

A further object is to provide an improved melt for nodular cast iron.

A further object is to decrease the degree of temperature control necessary in the melting of cast iron.

Other objects and advantages of the present invention will be apparent to those skilled in the art from the following description taken in conjunction with the drawings in which:

FIG. 1 is the structure at 100 diameters of a cast iron heated to 2600 F characterized by normal random graphite and areas of undercooled graphite;

FIG. 2 is the structure at 100 diameters of the same cast iron heated to a temperature of 2800 F., characterized by undercooled graphite type D and, also, type E of the interdendritic variety;

FIG. 3 is the structure at 100 diameters of a cast iron which has been heated to 2900" F., after adding onequarter percent silicon carbide. Characterized by normal flake graphite and some undercooled graphite, but no type B, interdendritic undercooled graphite;

FIG. 4 is the structure at 100 diameters of a cast iron which has been heated to 3000 F. after adding onequarter percent silicon carbide, characterized by normal flake graphite and a very slight tendency to interdendritic graphite;

FIG. 5 is the chill wedges test on a cast iron heated to from 2600 F. in increments of 100 F. to 3000 F.,

3,764,298 Patented Oct. 9, 1973 showing increase in cheill as the superheat temperature is increased;

FIG. 6 is the chill wedge test on a cast iron heated after adding one-quarter percent silicon carbide from 2600 F. in increments of F. to 3000 F., showing practically constant chill at all temperatures of superheat.

In the melting of cast iron in an electric furnace it has long been recognized that severe changes in the chill propensity and the resultant structure occur as the superheating temperature is raised. This happens in all electric furnaces capable of a high superheat, regardless of whether they are of the direct arc, the indirect arc, the low frequency channel type or the low, medium or high frequency coreless type.

It is found, in general, that the effects of superheating are more pronounced and more rapid in those electric furnaces that have high power inputs and where the stirring action on the bath is more severe. In the furnaces like the electric arc furnaces, for example, a very high local superheat occurs in the vicinity of the electrodes and this has a very definite effect on the quality of the melt. As a bath of cast iron of given composition is heated above its melting temperature and, finally, above its normal pouring temperature in an electric furnace, the first noticeable change in the characteristics of the iron is that an increase in the chill value of the bath occurs. This is depicted in FIG. 5 which shows the increase in chill value with superheating temperature of a typical Class 40 gray cast iron.

It is not known why this chill increase occurs, because in many cases there is actually very little change in the chemical composition of the bath. It is argued by most of those skilled in the art, that this increased chill value occurs due to coagulation and floating up out of the bath of the silicate slimes or slags which usually have some effect on the nucleation of the cast iron. Removal of these nuclei results in an increased chill value.

Along with this change in chill value and quite often, without any appreciable change in chill value, it is found that the bath exhibits an increased tendency to solidify with graphite in the undercooled condition, as the degree of superheating increases. Such undercooled graphite may or may not be desirable and, in the manufacture of high strength cast iron, it is usually undesirable as it cannot be completely removed by subsequent nucleation or graphitizing additions, which are customarily added to the molten metal before casting.

It has also been noted that cast iron which is highly superheated, while it does exhibit increasing fluidity, due to the fact that it is heated further above the solidification point, does not exhibit a fluidity increase which is commensurate with the degree of superheating. As the superheating increases, particularly with certain bath compositions, it is often found that a slight decrease in fluidity occurs.

All of these factors make it mandatory to exactly control the degree of superheat that is permissible with a cast iron bath. Failure to do this will give poor quality metal, exhibiting poor graphite structures, hard white edges, misruns and the like.

On the other hand, it has also been found, particularly in charges which have a high graphitic carbon content, that heating to a limited degree does not always allow complete solution of the charged graphite into the melt. This has been found to lead, also, to a relatively poor graphite structure in castings produced from such a melt where limited superheating only is allowed.

From the practical standpoint of having a high temperature in the bath which, in turn, allows a longer time for handling the metal, is a much more desirable condition and the practice, therefore, in general, is to superheat the bath to as high a degree as possible, at the same time trying to avoid the production of undercooled graphite in excess or high chill values in the metal.

In connection with the undercooled graphite, it appears that undercooled graphite is quite normal in an electric furnace melt, but that the interdendritic or type E graphite, which only seems to occur at or above a temperature of 2750 F., is more harmful as it does not respond completely to subsequent nucleation, which is a common practice to those skilled in the art.

In the manufacture of nodular cast iron, it has also been found that a low chill value in the initial bath is extremely desirable. Such a low chill value gives an increased nodule count with better mechanical properties and allows, also, the production of a good nodular graphite cast iron, with normal additions of nodularizing agents such as magnesium and cerium.

The use of a bath, having a high chill value, leads to free carbides in the final castings poured from such a bathand leads, also, to a low nodule count and a somewhat inferior shaped nodule.

Where cerium is used as a nodularizing agent, it is found that a high chill bath will produce extremely stable carbides on the addition of cerium and these carbides call for excessively long annealing temperatures in the final castings made from such a bath.

In general, therefore, cast iron baths made in electric furnaces, whether they be used for castings containing flake graphite or for castings containing nodular graphite, must be produced with a relatively low chill value for the particular composition of the bath. Allowing the development of higher chill values than those expected from a given composition will lead to inferior metal for the reasons given above.

Particularly important in this regard is to exercise absolute control and limit the superheating of the bath to a figure somewhere less than 2850 F. in most cases and quite often to a value of not more than 2800 F.

I have discovered that the presence of a small quantity of silicon carbide in the bath of molten cast iron will allow much higher superheating temperatuers to be developed, without the deleterious effects of superheating becoming evident.

FIG. 6 in the specification shows the chill value of a bath of Class 40 cast iron to which one-quarter percent of silicon carbide, in granulated form, was added immediately after melting, but before superheating. Superheating this bath to temperatures as high as 3000 F. did not increase the chill value over what it was at superheating temperatures as low as 2600" F.

FIG. shows the chill value of the same cast iron but to which no addition was made prior to superheating.

FIGS. 1 and 2 illustrate the structure of a typical Class 40 iron, which was heated to 2600 F. in FIG. 1 and to 2800 F. in FIG. 2. Portions of the bath were cast into test bars, which were subsequently examined for microstructure. In the casting of the 2600 F. superheating, illustrated in FIG. 1, the structure consisted of normal random flake graphite, together with some undercooled graphite, which would be typical of such an iron cast in the uninoculated condition from such a superheating temperature.

FIG. 2 illustrates the same iron Which has been superheated to a higher temperature, namely, 2800 F. In this case the structure taken from a representative test bar, contains a fairly high proportion of interdendritic graphite, along with normal undercooled graphite. The appearance of interdendritic graphite is typical in cast iron melts heated to temperatures in the vicinity of 2800 F.

Heating these irons to still higher temperatures will result in an increase in the amount of interdendritic graphite and, also, in the appearance of free carbides in the structure.

FIGS. 3 and 4 represent a similar Class 40" cast iron superheated to temperatures of 2900 F. and 3000 F. respectively. In this casean addition of one-quarter percent of granulated silicon carbide was made to the bath immediately after meltdown. Portions of the bath, taken and poured into representative test bars showed that the structure in the case shown in FIG. 3, where the heating temperature was 2900 F., consisted of normal random flake graphite, along with undercooled graphite. At a superheating temperature of 3000- F., shown in FIG. 4, the structure consisted of normal random flake graphite, with only very light traces of interdendritic undercooled graphite.

This clearly illustrates that silicon carbide is elfective in preventing the formation of interdendritic graphite, even at superheating temperatures as high as 3000 F. Where silicon carbide is not present, these undesirable structures may occur at temperatures as low as 2800 F.

I find that the amount of silicon carbide necessary to perform the process of my invention may be as little as one-eighth percent by weight of the bath or it can be as high as two percent, or more, by Weight of the bath. I usually prefer to add a smaller amount, such as one-quarter percent by weight of the bath, because this is normally completely eflFective and, in the interests of economy, would be a more desirable addition. I also prefer these lesser amounts, because they do not appreciably change the composition of the bath.

As a typical example, when adding one-quarter percent by weight of silicon carbides of approximately mesh, I find that the chemical changes and the other changes involved in the metal are as indicated in the following table:

A2,600 F. B2,700 F. C2,800 F. D2,900 F. E3,000 F.

Percent:

Of particular interest here is the relatively constant cell count, which was obtained in samples poured from these baths at different superheating temperatures. The slight increase in cell count at the highest superheating temperatures is presumably due to some solution of the silicon carbide in the melt at these temperatures and, without limiting our invention to an exact theory, we believe that it is this solution which prevents some of the ill effects which would normally be associated with superheating at these temperatures.

While I prefer to use granular silicon carbide, merely because it is easier to add to the bath, I have also found that lumps of silicon carbide are quite acceptable, because solution of the silicon carbide, which is quite refractory, is slow and as this solution occurs from the surface of the silicon carbide, it can be quite efiective. Because of this, I have found, also, that silicon carbide in lump form may be added directly to the cold charge in the electric furnace. It persists throughout the melting cycle and only dissolves in the melt at high superheating temperatures, where it is needed to prevent chill increase and the like. I would prefer, however, to add this silicon carbide in relatively small amount directly to the molten bath, because, by doing this, it is possible to achieve better control of the chemistry of the bath.

Although this invention has been described in its preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example and that numerous changes in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and the scope of the invention as hereinafter claimed.

What is claimed is:

1. A method of melting cast iron in an electric furnace at high temperatures consisting essentially of the steps of melting a cast iron charge in the presence of a small but effective amount of silicon carbide and superheating the melt to any desired temperature between about 2750 F. and 3000 -F. whereby said silicon carbide acts to prevent an increase in the chill value of the bath during superheating and to decrease the amount of undercooled graphite in the structure of the cast iron cast from said melt.

2. A method as claimed in claim 1, wherein said small but etfective amount of silicon carbide is in the range of from A; of one percent to two percent based on the weight of iron.

3. A method as claimed in claim 2, wherein said silicon carbide is added to the metal after it becomes molten.

4. A method as claimed in claim 2, wherein said silicon carbide is added to the metal before it becomes molten.

References Cited UNITED STATES PATENTS Becket 7511 Becket 7511 Baraduc-Muller 75--53 Brown 75130R Brown 75130'R Erasmus 7553 Moore 75130 R Fischer 7553 Drenning 7553 Curry 75130R US. Cl. X.R. 

